Mobile telephony systems, include user equipment, such as mobile handsets, have undergone rapid development through a number of generations. In the mobile telephony system the user equipment communicates via wireless links to a network of base stations connected to a telecommunications network. The initial deployment of mobile telephony systems, using analogue modulation for communication, was superseded by second generation digital systems, which are themselves currently being superseded by third generation digital systems such as UMTS and CDMA. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposal by the Third Generation Partnership Project of the so-called Long Term Evolution system, often simply called LTE, which offers potentially greater capacity still, by the use of wider frequency bands, spectrally efficient modulation techniques and, potentially, the exploitation of spatially diverse propagation paths to increase capacity (Multiple In Multiple Out).
Distinct from mobile telephony systems, wireless data access systems have also undergone development. Wireless data access systems were initially aimed at providing the “last mile” (or thereabouts) connection between user equipment at a subscriber's premises and the public switched telephone network (PSTN); the user equipment, typically, being a terminal to which a telephone or computer is connected. The WiMax standard (IEEE 802.16) has provided a means for such terminals to connect to the PSTN via high data rate wireless access systems.
Whilst WiMax and LTE have evolved via different routes, both can be characterised as high capacity wireless data systems that serve a similar purpose, typically using similar technology, and in addition both are deployed in a cellular layout as cellular wireless systems. Typically such cellular wireless systems comprise user equipment such as mobile telephony handsets or wireless terminals, a number of base stations, each potentially communicating over what are termed access links with many user equipments located in a coverage area known as a cell, and a two way connection, known as backhaul, between each base station and a telecommunications network such as the PSTN.
As the data capacity of cellular wireless systems increases, an increased demand is placed on the capacity of the backhaul, the connection that has to convey the wireless-originating traffic to its destination, often in an entirely different network. For earlier generations of cellular wireless systems, the backhaul has been provided by one or more connections leased from another telecommunications operator (where such a connection exists near to the base station). However, increasing data rates increases the number of leased lines required to convey the data. Consequently, the operational expense associated with adopting multiple leased lines has also increased, making this a potentially expensive option for high capacity systems. As an alternative to leased lines, dedicated backhaul links can be provided by a variety of methods including microwave links or optical fibre links. However each of these methods of backhaul has associated costs. Dedicated fibre links can be expensive in terms of capital expense due mainly to the cost of the civil works in installation, and this problem is especially acute in urban areas. Microwave links also involve the capital expense of equipment and require expert installation due to narrow beam widths leading to the requirement for precise alignment of antennas.
As an alternative to the provision of a dedicated backhaul link for each individual base station, it is possible to use the radio resource of the cellular wireless system to relay backhaul traffic from one base station to another. Typically, the base station using the cellular radio resource for backhaul is a small low power base station with an omnidirectional antenna known as a relay node. Such a system can be used to extend the area of cellular wireless coverage beyond the area of coverage of conventional base stations that are already equipped with a dedicated backhaul.
FIG. 1 illustrates a conventional in-band wireless cellular network 2; in this instance, base stations, or relays, 4a-4d are connected, through wireless channels, to an aggregation node 6. The aggregation node 6 also acts as a base station for user terminals and is included in the same cellular planning layout as the base stations 4. The aggregation node 6 is connected, for example by a microwave or fibre link 10, to a gateway 12. The gateway 12 is then, in turn, connected to a telecommunications network (not shown) again, possibly also by using wireless channels. This architecture re-uses the radio equipment and spectrum allocation already provided for data links to users, also to provide the backhaul communication between a group of base stations.
It is usual for the channels connecting the base stations 4a-4d to the user equipment terminals and connecting the aggregation node 6 to the user equipment terminals to be provided using either a Time Division Duplexing (TDD) or a Frequency Division Duplexing (FDD) system. Often different operators within the same coverage area will have one, or both, systems available to user equipment connecting via one of the base stations 4 or 6.
In TDD, each channel having a predetermined frequency range is divided into a number of time frames; each frame being subdivided into a plurality of timeslots. Some of the timeslots in each frame are designated for uplinking and some are designated for downlinking, with each piece of user equipment being allocated particular uplink and downlink timeslots for a particular communication session. Of course, different operators will, in general, have different frequency channels allocated to them.
In FDD, two bands of frequencies are available as communication channels, one for uplink (meaning a data link from the user terminal to the base station) and the other for downlink (meaning a data link from the base station to the user terminal). For a particular communication session with user equipment, the operator will allocate a number of frequency channels from the uplink band as an uplink channel and a number of frequency channels from the downlink band as a downlink channel to that user equipment. The user equipment will then transmit and receive data using those particular frequency channels for the duration of a communication session. Different pieces of user equipment may share the same uplink and downlink channels by being assigned different spreading codes or OFDM sub-carriers to allow their data transmissions to be distinguished.
Conventionally, a base station and aggregation node in a wireless network will either operate TDD mode or FDD mode. This limits the ability of the base stations to serve user equipment supported by different operators. It also restricts the use of in-band backhaul within the network.